Dual mode scanner/tracker

ABSTRACT

The beam of a laser radar is moved over the field of view by means of a  p of scanner/trackers arranged in cascade along the laser beam. One of the scanner/trackers operates at high speed, with high resolution and a narrow field and is located in the demagnified portion of the laser beam. The other scanner/tracker operates at low speed with low resolution and a wide field and is located in the magnified portion of the laser beam. The two scanner/trackers complement each other to achieve high speed, high resolution scanning as well as tracking of moving targets. A beam steering telescope for an airborne laser radar which incorporates the novel dual mode scanner/tracker is also shown.

The Government has rights in this invention pursuant to contractDAAB07-76-C-0920, awarded by the Department of the Army.

BACKGROUND OF THE INVENTION

This invention relates to optical radar of the type which illuminatestargets by means of a laser beam and derives target information from thereflected laser beam. Such radars usually include a scanning andtracking capability. The scanning system moves the transmitted laserbeam over the field of view, usually in some systematic manner, forexample with a sawtooth scan system of the type used in television orwith spiral type scanning. If such a radar is provided in addition witha tracking capability for moving targets, the scan format must berandomly programmable so that random target movements can be followed.

Scanner/trackers for laser beams may include a coarse scanner, forexample a wide angle, low speed, low resolution scanner; with a narrowfield, high resolution, high speed dither scanner in series with thecoarse scanner. With such a dual mode scanner/tracker system, the coarsescanner may for example scan in a sawtooth fashion with gaps between thescanning lines, with the high speed dither scanner filling in the gaps.Thus the two scanners complement each other. In the tracking mode bothof these scanner/trackers move in a programmed coordinated manner toachieve target tracking.

Scanners of this type usually achieve laser beam movement by means ofmoving optics such as rotating prisms or wedges through which the beampasses or electrically driven moving mirrors from which the beam isreflected. High speed tracking, such as is required for theaforementioned dither scanner/tracker requires extremely high power ifthe moving optics are located at a point where the laser beam has itslargest diameter. Laser radars normally include a means to expand thebeam diameter before transmission to improve angular resolution orprovide greater range.

SUMMARY OF THE INVENTION

The present invention comprises a dual mode scanner/tracker in which thehigh speed, narrow field, high resolution, scanner/tracker is located inthe demagnified portion of the laser beam where power requirements forthe smaller moving optics are moderate, with the coarse, wide field, lowresolution scanner/tracker located in the magnified or large diameterportion of the laser beam.

The invention also comprises a beam steering telescope adapted formounting on an aircraft and comprising a rotating turret which includesa dual mode scanner/tracker of the type described above, together withthe beam expanding telescope. The beam steering telescope is thuscapable of scanning its field of view 360° in azimuth while the dualmode scanner/trackers are operating.

It is thus an object of the invention to provide a dual modescanner/tracker for an optical radar which includes a high speed, highresolution, narrow field scanner/tracker located in the demagnifiedportion of the laser beam of said radar, and a wide field, low speed,low resolution scanner/tracker located beyond the beam expandingtelescope of said optical radar, whereby said two scanner/trackers aredesigned to coordinate with each other to provide high speed highresolution scanning of the radar's wide field of view and efficienttracking of randomly moving targets.

Another object of the invention is to provide a beam steering telescopefor airborne radar which includes a rotatable turret including a dualmode scanner/tracker and a beam expanding telescope, with the beamexpanding telescope mounted between the two scanner/trackers, one ofsaid scanner/trackers being a high speed, high resolution, narrow fieldscanner/tracker which is located in the demagnified or narrow beam sideof said beam expanding telescope, and the other of said scanner/trackersbeing a wide field low resolution, low speed scanner/tracker located onthe other side of said beam expanding telescope in the magnified or widebeam area of said laser radar.

A still further object of the invention is to provide a scanner/trackerfor an optical radar which achieves high resolution, high speed, wideangle, and low access time scanning and tracking with minimum powerrequired to operate the scanning mechanisms.

These and other objects and advantages of the invention will becomeapparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the concept of the novel dualmode scanner/tracker.

FIG. 2 shows one way in which the novel concept of FIG. 1 can beimplemented.

FIG. 3 shows additional details of the apparatus of FIG. 2.

FIG. 4 is a pictorial view of a beam steering telescope in which thenovel dual mode scanner/tracker is integrated with the beam expandingtelescope of the optical radar.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The diagram of FIG. 1 shows a portion of an optical radar includingnarrow field scanner/tracker 5, a beam expanding telescope 7 whichreceives the output of scanner/tracker 5 and applies the expanded laserbeam 13 to wide field scanner/tracker 9, which radiates the laser beaminto space, and receives laser target echoes. The narrow fieldscanner/tracker receives the narrow transmitted laser beam from theoptical radar transceiver circuitry to the left thereof, not shown, andalso applies the target echo signals passing therethrough to said radarcircuitry.

A known method of achieving efficient scanning/tracking of optical radarbeams is to provide two scanner/trackers in series or cascade along thebeam with one scanner/tracker having a wide field of view, for example60°, low resolution and low scanning speed. Such a scanner/tracker mustnecessarily be located in the wide or expanded beam region of the laserbeam. The low resolution and low scanning speeds result in moderatepower requirements for moving the necessarily large optics over such alarge field of view. The low speed and low resolution of such ascanner/tracker can be enhanced by a narrow field, high resolution, highspeed scanner/tracker in series therewith, with the beam movements ofthe two scanner/trackers designed to complement each other. For example,the narrow field scanner/tracker may have a 1° field of view, referencedto the optical system output, which means that this scanner/tracker iscapable of high speed, high resolution movement of the beam over thisangle anywhere within the large field of view of the wide fieldscanner/tracker. In FIG. 1, the angle 15 at the radar system outputindicates the overall wide field of view of the radar with the smallerangle 17 representing the field of view due to the action of the narrowfield scanner/tracker. For example the angle 15 may be 60° and the angle17, 1°. The narrow field scanner/tracker could be located in the wide orexpanded portion of the laser beam to the right of the beam expandingtelescope, however the size of the moving optics required for such alocation would have to be at least equal to the beam size. Since momentsof inertia of reciprocating or rotating mirrors or prisms go up with thesquare of the diameters thereof, the power requirements for achievinghigh speed, high resolution performance even over a small angle can beprohibitive. Significant power and consequent weight saving can berealized by locating the narrow field scanner/tracker in the narrow ordemagnified portion of the laser beam, to the left of the beam expandingtelescope as shown in FIG. 1. At this location, the scanner/trackeroptics can be scaled down to the approximate diameter of the narrow ordemagnified laser beam, however the scanned field of view must beincreased by the magnification of the beam expanding telescope. Forexample, if the telescope 7 of FIG. 1 has an afocal magnification of 20,the beam 13 in the output thereof will have a diameter 20 times thediameter of the beam 11 applied thereto from scanner/tracker 5, but thescan field of the expanded beam 13 will be reduced from the angle 12 atthe telescope input, also by a factor equal to the telescopemagnification. Thus if the scan field or angle 17 at the radar's outputis to be 1° in this example, the scanner/tracker 5 would be required toscan the beam 11 over the angle 12 equal to 20°. Even with this largerscan field, the reduced size of the optics for the narrow fieldscanner/tracker results in power and weight savings.

The diagram of FIG. 2 is one example of how the concept of FIG. 1 can beimplemented. In FIG. 2, a portion of the optical radar circuitry isshown, including an optical duplexer 21 which directs the target echoes37 to a receiver, not shown, and passes the narrow transmitted laserbeam 19 to the narrow field scanner/tracker 25 via quarter wave plate23. The high speed, narrow field, high resolution scanner/tracker 25utilizes a pair of electrically driven reciprocating mirrors 27 and 31which rotate around orthogonal axes 29 and 33 respectively to producescanning or tracking with a bandwidth from dc to over 1.0 kHz. Themirrors are electrically driven as indicated by the arrows 35 labelled"Programmable Drives" and readouts 39 are provided for indicatinginstantaneous mirror positions. Such a scanner/tracker may have a 1°scan field, referred to the radar system output, with 3×10³ elements perfield (circular field with a diameter of 64 elements) and frame time of1/30 second. The effective aperture may be 0.5 cm. with 0.02° randomaccess resolution and 0.26 milliseconds random access time.

The output beam 40 of scanner/tracker 25 is applied to the input of thebeam expanding telescope 43 via relay optics 41. The relay optics may berequired to keep the wide scan angle output of the scanner/tracker 25within the small entrance pupil of telescope 43. The details of therelay optics and telescope are illustrated in more detail in FIG. 3. Theoutput beam 45 of telescope 43 will be a wide beam, for example 10 cm.in diameter if the narrow beam 40 is 0.5 cm. in diameter and telescope43 has a magnification of 20. Also the relatively wide scan field angleof the beam 40 will be reduced by this factor of 20, for example from20° to 1° in the telescope output.

The wide field scanner/tracker 47 may comprise a pair of in-linerotating wedges or prisms 49 and 51 with apertures or diameterssufficient to accommodate the magnified scanned laser beam appliedthereto from telescope 43. Such a rotating wedge scanner can have atotal field of view of 60° with 0.6° random access resolution and a 20milliseconds response time. The wedges 49 and 51 are separately drivenas indicated by the arrows 53 labelled "Programmable Drives" and eachhas separate readouts 55 for indicating the position thereof. Theseversatile programmable scanner/trackers may be provided with a 16 bitoptical shaft encoder as part of the readout system thereof foraccurately monitoring the instantaneous scanner line of sight to withinthe diffraction limited resolution of the radar, which is approximately250 microradians. Further details of such rotating wedge programmablescanner/trackers will be found in a co-pending application Ser. No.377,727, entitled PROGRAMMABLE SCANNER/TRACKER, Filed on May 13, 1982.

Dual in-line optical wedges may also be used for the narrow field highspeed scanner 25, because of their superior performance in highvibration environments, for example such as would occur in an opticalradar installed in a helicopter. Small aperture wedges for such anapplication would be competitive in frequency response to thereciprocating mirrors shown, but would require more signal conditioningto realize a tracking capability because of their non-linear transferfunction.

Also, rather than using a pair of reciprocating mirrors for thescanner/tracker 25, single mirror could be used, mounted on dual gimbalswhich are separately driven by the x and y scanning signals. Thisarrangement may obviate the necessity for the relay optics 41.

The details of the relay optics 41 and the telescope 43 are shown inFIG. 3. As can be seen the beam expanding telescope may comprise merelya pair of lenses 63 and 65 arranged along the optical axis O--O. A beamdirected into ocular or entrance pupil 63 will emerge from the objectivelens 65 expanded in diameter and with a reduced scan field, as explainedabove. The relay optics 41 may comprise, for example, merely a singlepositive lens 61 positioned so that the narrow laser beam 40 fromscanner/tracker 25 is concentrated at the entrance pupil 63 of thetelescope 43, indicated by the converging rays 42.

FIG. 4 shows a beam steering telescope which embodies the dualscanner/tracker of the present invention mounted in a rotatable turret70 which is mounted on the underside of an aircraft 71. In thisembodiment the narrow field scanner/tracker is integrated with the beamexpanding telescope to reduce the number of optical components. Thenarrow laser beam 79 is applied to device 81 which includes both thenarrow field scanner/tracker as well as the relay optics, if necessary,and the ocular lens of the beam expanding telescope, such as lens 63 ofFIG. 3. The beam 83 emerging from device 81 is reflected from fixed 45°mirror 85 which is mounted along the axis of rotation 75 of turret 70.The beam 87 then passes through the telescope objective lens 89 and isturned by another 90° by means of a second 45° mirror 91. The beam 88then passes through the wide angle scanner/tracker which may comprisethe two rotating wedges 95 and 97 plus ancillary apparatus, not shown,and emerges into space as the scanning beam 77. The arrow 73 representsthe rotation of the turret around the axis 75.

While the invention has been described in connection with illustrativeembodiments, obvious variations therein will occur to those skilled inthis art, accordingly the invention should be limited only by the scopeof the appended claims.

What is claimed is:
 1. A dual mode scanner/tracker system for an opticalradar, said radar comprising a laser beam having magnified anddemagnified portions, a first programmable scanner/tracker located insaid demagnified portion of said laser beam, said first scanner/trackerbeing high speed, high resolution and having a narrow field, a secondprogrammable scanner/tracker located in the magnified portion of saidlaser beam, said second scanner/tracker being low speed, low resolutionand having a wide scan angle, said two scanner/trackers being eachdriven by programmable drives which complement each other to produceefficient scanning of said laser beam and efficient tracking of randomlymoving targets in the field of view of said optical radar.
 2. The systemof claim 1 wherein said first and second scanner/trackers are separatedby a beam expanding telescope, said beam expanding telescope comprisingan entrance pupil.
 3. The system of claim 2 wherein said firstscanner/tracker comprises a pair of orthogonally mounted, electricallydriven reciprocating mirrors which produce linear sawtooth scanning witha bandwidth from zero to 1.0 kHz, relay optics located at the output ofsaid first scanner/tracker arranged to constrain the output of saidfirst scanner/tracker to the said entrance pupil of said beam expandingtelescope, said second scanner/tracker comprising a pair of in-line,transparent, rotating wedges or prisms, said wedges having separateprogrammable drives and separate readouts for indicating theinstantaneous wedge positions.
 4. A beam steering telescope forming partof an airborne optical radar, comprising; an aircraft, a rotating turretmounted on the underside of said aircraft, said beam steering telescopecomprising a first high speed, high resolution and narrow fieldprogrammable scanner/tracker located in the demagnified portion of thelaser beam of said optical radar, the output of said firstscanner/tracker being applied to a beam expanding telescope the outputof which is a magnified laser beam, a fixed 45° mirror arranged todirect the said output of said beam expanding telescope along the axisof rotation of said turret to another fixed 45° mirror which directssaid expanded beam through a second programmable scanner/tracker whichhas a wide field, low resolution, and low scanning speed.
 5. The beamsteering telescope of claim 4 wherein said first scanner/tracker isintegrated with said beam expanding telescope.
 6. The beam steeringtelescope of claim 4 wherein said first scanner/tracker comprises a pairof orthogonally mounted electrically driven reciprocating mirrors andsaid second scanner/tracker comprises a pair of transparent rotatingwedges through which the said magnified laser beam passes.
 7. A dualmode scanner/tracker system for an optical radar, said radar comprisinga laser beam having magnified and demagnified portions, a firstprogrammable scanner/tracker located in said demagnified portion of saidlaser beam, said first scanner/tracker being high speed, high resolutionand having a narrow field; a second programmable scanner/tracker locatedin the said magnified portion of said laser beam, said secondscanner/tracker being lower speed, lower resolution and having a widerscan angle, relative to the same characteristics of said firstscanner/tracker.